You've done things in wrong way
On the host, edit the /etc/apparmor.d/lxc/lxc-default file
It should look as follow:
Note: ... means any other thing that was already present in your /etc/apparmor.d/lxc/lxc-default file before or after.
Once done, reload apparmor:
Then, rerun the i-MSCP installer in the LXC container (perl imscp-autoinstall -d).
See also